This invention relates to a method of and apparatus for removal of fine particles from a gas stream, for example fly ash particulates from the gaseous emissions of a coal-fired electrical generating power station.
Along with rapid industrial growth over the last two decades, there has been an alarming increase in the discharge of harmful pollutants into the environment. Unfortunately, the necessary pollution abatement technology to minimize, or eliminate the discharge of industrial waste material and its harmful effects has not kept pace with overall technological growth. To stimulate the needed pollution control innovations, stringent standards have been imposed on industry requiring the reduction or total elimination of particulate discharge in the atmosphere.
Schwab et al U.S. Pat. Nos. 4,093,430 and 4,110,806 describe a recent technological advancement in air pollution control, in particular the removal of fine particles of 0.1 .mu.m to 3.0 .mu.m diameter. These patents describe a high intensity ionization system (hereafter referred to as "HII system") wherein a disc-shaped discharge electrode is inserted in the throat of a Venturi diffuser. A high D.C. voltage is imposed between the discharge electrode or cathode and the Venturi diffuser, a portion of which acts as an anode. The high voltage between the two electrodes and the particular construction of the cathode disc produces a stable corona discharge therebetween of a very high intensity. Particles in the gas which pass through the electrode gap of the Venturi diffuser are charged to very high levels in proportion to their sizes. The entrained particulates are field charged by the strong applied field and by ion impaction in the region of corona discharge between the two electrodes. The high velocity of the gas stream through the Venturi throat prevents the accumulation of space charge within the corona field established at the electrode gap and thereby improves the stability of the corona discharge between the two electrodes.
In the further HII improvement of Satterthwaite, U.S. Pat. No. 4,108,615, jets of cleaned air are introduced along the anode wall to prevent particle deposition thereon and to mechanically remove excess deposits from the anode, thus preventing the onset of back corona.
Although the HII device can be used as a precharger for a variety of particle collection devices including fabric bags, Venturi scrubbers, fixed and fluidized bed collectors, in its principal use the HII device is used as a precharger for an electrostatic precipitator assembly. In this use, the entire HII system is functionally analogous to a conventional two-stage electrostatic precipitator,, although the HII operates as a much more effective precharger than the wire-plate ionizer stage of the two-stage precipitator used, for example, as a room air purification device.
In the drawings, FIGS. 1-3 represent prior art apparatus. As illustrated in the cross-section elevation view of FIG. 1, each HII comprises a tubular Venturi-shaped anode 10a through 10f and a disc-shaped cathode 11a through 11f positioned within the anode. FIG. 2, a cross-section end view of FIG. 1 taken along line A--A, illustrates alignment of the individual HII devices with their respective axes parallel to one another, to thereby present a honeycomb-like array of flow passages to the particulate laden gas stream. The discharge end of the HII is then aligned directly in front of an electrostatic precipitator, shown as FIG. 3, a cross-sectional end view taken along line B--B of FIG. 1.
The electrostatic precipitator comprises a series of grounded, equally spaced parallel plates 12a through 12f which serve as the collecting electrodes. Spaced uniformly between these plates are electrically charged wires 13 which function as discharge electrodes and thereby establish an electric field between the wires and plates. As illustrated, there are four wires 13 between each pair of plates. Charged particles from the HII unit which enter the electrostatic precipitator are forced by the electric field to an appropriate electrode and are thereby collected. The trapped solids are thereafter removed by mechanically rapping the collector electrodes. The collected particulates then fall by gravity into a collection hopper located in a chamber directly below the electrostatic precipitator. A detailed description of a standard electrostatic precipitator is presented in Chapter 2 of Industrial Electrostatic Precipitation, Harry J. White, Addison-Wesley Publishing Company, Inc., Reading, Massachusetts, 1963.
While a system incorporating the HII device as described in the aforementioned Schwab et al patents represents a significant improvement relative to prior art particulate collection assemblies, it nonetheless has a significant drawback which impairs the overall particle collection efficiency of the HII--electrostatic precipitator assembly.
In order to insure optimum operation, a uniform flow of gas must be established through the electrostatic precipitator assembly. One way to establish uniform gas flow without significantly increasing the pressure drop of the entire system is to provide a sizeable plenum chamber between the HII outlet and the electrostatic precipitator inlet. A uniform flow of gas through the electrostatic precipitator assembly with a uniform particle concentration is essential for a variety of reasons. First of all, in order to insure that the collection area is being used to its fullest potential, it is necessary that no particulate loading gradient or particle size stratification exist between neighboring collector passages. Such a loading gradient and size stratification would reduce the overall effectiveness of those collector plates which have to remove a non-proportionally large share of the entrained solids. As a result, it is necessary that a gas having a uniform concentration of particulates be uniformly distributed among the various parallel flow channels provided by the electrostatic precipitator design.
Another reason why uniform gas flow and particle concentration is important is to minimize scouring and reentrainment of collected particulates. To accomplish this, it is necessary to keep all of the local gas velocities near each of the collector electrodes as close to the mean flow velocity as possible. This condition can best be satisfied by insuring that the gas is initially distributed to the various flow channels of the electrostatic precipitator assembly as uniformly as possible. This will provide nearly identical localized gas velocities. Such uniform velocities also insure that particle residence times are uniform throughout. The resultant migration velocity of a particulate between any two electrodes in an electrostatic precipitator is primarily determined by the drag experienced by the particle while moving toward the electrodes, which opposes collection, and the electric field force on the particle normal to the electrodes which contributes to collection. A particle's residence time is directly influenced by the velocity of the gas stream within which it is entrained. High localized velocities due to maldistribution of flow, therefore, will reduce the residence time available for particle migration and accordingly reduce collection efficiencies; while uniform velocities will insure that the electrostatic precipitator assembly as a whole operates with the most optimal collective forces.
A final reason for achieving uniform gas flow and particle concentration is that non-uniform conditions may under certain circumstances, lead to a localized sparking between neighboring discharge and collector electrodes. Sparking is the rapid breakdown of the electric field between neighboring electrodes which serves to significantly degrade collection efficiency. Sparking may discharge already charged particulates, and may even produce particles of an opposite polarity to those produced by the HII device. In either case, overall efficiency suffers.
However, the mere presence of the plenum chamber for insuring a uniform flow itself causes additional problems in the operation of the integrated HII-electrostatic precipitator system for which the prior art has not yet provided solutions. The principal problem involves the so-called space charge phenomenon. In broad terms, "space charge" refers to a perponderance of negative ions within any given portion of the plenum chamber between the HII assembly and the electrostatic precipitator assembly. When an inordinately large accumulation of negative ions occurs in the plenum chamber of the HII-electrostatic precipitator assembly, there is a high likelihood for localized discharge or neutralization of the charged particles by arcing or spark-over to grounded protrusions inside the plenum chamber. The discharge produces neutral or even positively charged particles which cause a net reduction in the overall charge on entrained particles and inferior operation. The extent or degree of the accumulation of "space charge" varies directly with the size of the plenum chamber.
This space charge effect has a major impact on the overall collection efficiency of particles, especially fine particles. In order to satisfactorily comply with air quality standards in the majority of the cases involving the removal of airborne particulates, a major portion of the fine particulates must be removed. Adequate removal of fine particles in an electrostatic precipitator requires that they be highly charged prior to entering the electrostatic precipitator assembly, so that the applied electric field force therein is sufficient to effect particle collection. The electrostatic precipitator alone cannot adequately provide the required charging to sufficiently remove fine particles.
It has been found that the HII is uniquely suited for providing the needed charge on fine particles. However, the effectiveness in fine particle collection gained by employing the HII is disadvantageously diminished when arcing or spark-over occurs. The charges on the fine particles are reduced or eliminated and some particles may avoid collection in the downstream electrostatic precipitator.
An object of this invention is to provide an improved high intensity ionization-electrostatic precipitation system for separation of fine particles from gas streams.
Another object is to provide an improved high intensity ionization-electrostatic precipitation assembly with reduced sparking and higher particle removal efficiency.
Other objects and advantages will be apparent from the ensuing disclosure and appended claims.